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Bureau of Mines Information Circular/1985 



Factors Affecting Respirable Dust 
Generation From Longwall Roof 
Supports 



By John A. Organiscak, Jeffrey M. Listak, 
and Robert A. Jankowski 




UNITED STATES DEPARTMENT OF THE INTERIOR 



75j 

'^INBS 75TH AVi^ 



Information Circular 9019 



Factors Affecting Respirable Dust 
Generation From Longwall Roof 
Supports 



By John A. Organiscak, Jeffrey M. Listak, 
and Robert A. Jankowski 




UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Model, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 





Library of Congress Cataloging in Publication Data: 



Organiscak, John A 

Factors affecting respirable dust generation from longwall roof 
supports. 

(Bureau of Mines information circular ; 9019) 

Bibliography: p. 16. 

Supt. of Docs, no.: I 28.27:9019. 

1. Coal mines and mining— Dust control. 2. Mine roof control. I. 
Listak, Jeffrey M, II. Jankowski, Robert A. III. Title. IV. Series: 
Information circular (United States. Bureau of Mines) ; 9019. 

TN295.U4 [TN312] 622s [622'. 42] 84-600328 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Survey description 2 

Longwalls with low concentrations of support-generated dust 3 

Geology 4 

Supports 5 

Operational procedures 5 

Longwalls with moderate concentrations of support-generated dust 5 

Geology 5 

Supports 6 

Operational procedures 7 

Longwalls with high concentrations of support-generated dust 7 

Geology 8 

Supports 9 

Operational procedures 9 

Factors affecting dust generated by supports 9 

Roof strength 9 

Depth of cover 10 

Shield design 10 

Other factors 10 

Support dust control technology used in the United States 11 

Water application 12 

Other dust control practices 12 

Conclus ions 15 

References 16 

Appendix A. — Data from longwall survey. 17 

Appendix B. — Dust-sampling strategy 19 

ILLUSTRATIONS 

1 . Typical longwall shield face 3 

2. Instantaneous roof support dust concentrations at longwall A 4 

3. Instantaneous roof support dust concentrations at longwall F 6 

4. Instantaneous roof support dust concentrations at longwall G 8 

5. Relationship between roof loading factor and average support dust concen- 

tration at surveyed longwall faces 10 

6. Relationship between panel depth and average support dust concentration 

at surveyed longwall faces 11 

7. Relationship between face air velocity and dust levels at the face 12 

8. Spray manifold located on support canopy 13 

9. Relationship between debris thickness on shield canopy and dust generated 14 

10. Effectiveness of shearer-clearer system 14 

11. Relationship between distance downwind of support movement and dust level 15 

TABLES 

A-1 . Numerical data from survey 17 

A-2. Descriptive data from survey 18 





UNIT OF MEASURE ABBREVIATIONS USED IN THIS 


REPORT 


ft 


foot psi 


pound (force) per 
square inch 


ft2 


square foot 






psig 


pound (force) per 


h 


hour 


square inch, gauge 


in 


inch pet 


percent 


mg/m^ 


milligram per cubic meter ton/ft^ 


ton per square foot 


mm 


millimeter ft/min 


foot per minute 


m/s 


meter per second L/min 


liter per minute 


ym 


micrometer 




NOTE.- 


-See appendix B for explanation of RAM units. 





FACTORS AFFECTING RESPIRABLE DUST GENERATION 
FROM LONGWALL ROOF SUPPORTS 

By John A. Organiscak, Jeffrey M. Listak, 
and Robert A. Jankowski 



ABSTRACT 

The Bureau of Mines conducted a survey of eight shearer longwall oper- 
ations to identify factors that affect respirable dust generation from 
longwall roof supports. The longwalls surveyed were in coal seams lo- 
cated in different geographic regions of the United States. Data were 
collected on mining (geologic) conditions, support design, operational 
characteristics, and amount of respirable dust generated from roof sup- 
ports. Analysis indicated that mining conditions are the main factors 
that affect the generation of dust during roof support movement. Both 
roof strength and depth of cover above the coal seam showed relation- 
ships with the amount of support dust generated. Several practices are 
currently employed to effectively control roof support dust; however, 
some of these controls are limited. More research and development is 
needed to improve dust control technology for longwall roof supports. 



^Mining engineer. 
^Supervisory physical scientist. 
Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 



INTRODUCTION 



Previous Bureau research on dust 
sources at shearer longwall operations 
has shown that roof support movement can 
generate a significant amount of respira- 
ble dust (_1).-^ Investigators found that 
as much as 31 pet of the respirable dust 
to which shearer operators were exposed 
was generated by the cyclic movement of 
longwall roof supports Q) . 

On most longwalls in the United States, 
the major source of dust is the cutting 
action of the shearer drums , and the ma- 
jor effort has been to control dust from 
this source. Thus, very little research 
has been done on the control of support- 
generated dust in the United States (J^). 
Some research has been done in Europe, 
but only limited technology has resulted. 

To fill this gap, the Bureau conduct- 
ed a recent study to gain a better 



understanding of conditions that contrib- 
ute to high levels of support dust on 
longwall mining operations. The objec- 
tive was to identify the inherent char- 
acteristics of longwalls having high 
levels of support-generated respirable 
dust. This report describes the study 
and presents the findings. 

The problem was addressed by conducting 
dust sampling at longwalls in the east- 
ern and western United States having a 
wide range of dust levels generated by 
support movement. The criteria used for 
assessing the origins of respirable dust 
were local geology, support design, and 
operational procedures. The data were 
collected and analyzed for correlations, 
in an attempt to identify the character- 
istics that influence dust generation. 



SURVEY DESCRIPTION 



Eight shearer longwall faces using 
shield supports were surveyed (fig. 1).^ 
The characteristics and dust concentra- 
tions of these longwalls are shown in ap- 
pendix A. The longwalls surveyed were 
located in seven coal seams (table A-2 in 
appendix A) at varying depths of cover, 
in different geographic regions of the 
United States. Geologic conditions were 
observed during the survey, and drill- 
core data from the vicinity of the panels 
were obtained from mine personnel. 

During the survey, four types of imme- 
diate roof were identified. Compressive 
strengths of these roof types were esti- 
mated (based on observation) for use in 
relating the support load exerted on them 
to the dust generated (_2 ) . The four roof 
types and their estimated compressive 
strengths were as follows: coal, 650 
psi; weak shale (soft), 4,000 psi; strong 

-^Underlined numbers in parentheses re- 
fer to items in the list of references 
preceding the appendixes. 

'^Frame and chock support faces were not 
included in this survey because only a 
limited number of these installations are 
in use, and their application in the 
United States is quickly diminishing. 



competent shale, 10,000 psi; and weak 
siltstone (soft), 4,000 psi. The esti- 
mated roof strength and roof type for 
each of the longwalls surveyed are given 
in tables A-1 and A-2, respectively. 

Supports used at the longwalls surveyed 
were either two- or four-legged shields. 
The pertinent characteristics of the 
shields were setting pressures, yield 
loads, support dimensions, and leg speci- 
fications. From these data, the average 
setting-load density exerted on the roof 
by the supports at each longwall was de- 
temnined. As an indication of the roof's 
susceptibility to crushing under the sup- 
port setting load, a roof loading factor 
was devised from a ratio of the average 
roof strength to the average support 
setting-load density. The lower the val- 
ue of this factor, the more likely the 
roof was to crush under set load. 

Dust samples were taken at each long- 
wall, and operational procedures were 
observed. Support dust was measured 
by using GCA^ Real-Time Aerosol Moni- 
tors (RAM's) and short-term gravimetric 

^Reference to specific products does 
not imply endorsement by the Bureau of 
Mines. 




FIGURE 1. - Typical longwall shield face. 



samplers immediately upwind and downwind 
of support movement along the face. (For 
details of the sampling strategy, see ap- 
pendix B) . Ventilation data were col- 
lected at each longwall. At some of the 
longwalls, the effect of ventilation on 
the dilution and diffusion of support 
dust at various distances downstream of 



support movement was determined from RAM 
measurements. (See appendix B.) Addi- 
tional information about operational pro- 
cedures was collected, including support 
advance practices, support dust control 
practices, and horizon control of the 
roof and floor with the shearer. 



LONGWALLS WITH LOW CONCENTRATIONS OF SUPPORT-GENERATED DUST 



At two of the longwalls surveyed (long- 
walls A and B, as identified in appendix 
A), dust concentrations generated by sup- 
port movement were very low. The average 
respirable dust concentration was < 0.5 
RAM unit^ as measured with the RAM's and 

°Ram units are explained in appendix B. 



< 0.5 mg/m-' as measured with the gravi- 
metric samplers. 

An example of low dust concentrations 
generated by support movement along the 
face is shown in figure 2, a plot of the 
instantaneous measurements made at long- 
wall A. The difference between the imme- 
diate downwind and upwind concentrations 



h| 


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LU O 




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Z H 


1 


r=^ z 




h: uj 




CO o 





o 0. 



. 1 1 1 1 1 1 1 1 1 1 


1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 

KEY 
o Imnnediate downwind cone 
° Immediate upwind cone 

^ I//J Dust from support 
\ movement 

l^^Av downwind cone = 0.9\ £ 


! 1 1 I I J I -T— T— 

Av Upwind cone _ 


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1 1 1 


1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 


1 1 1 1 1 1 1 1 1 



30 



40 50 60 
-* — Headgate 



70 



80 90 100 

Tailgate — >- 
SUPPORT MOVED 
(Support identification number) 



10 



FIGURE 2. - Instantaneous roof support dust concentrations at longwoll A. 



measured during support movement indi- 
cated the dust concentration generated by 
the supports (the shaded area between the 
two curves). The average downwind dust 
concentration was 0.9 RAM unit, and the 
average upwind dust concentration was 0.5 
RAM unit, resulting in an average dust 
concentration of 0.4 RAM unit generated 
by the supports. The difference between 
the dust concentrations of the downwind 
and upwind gravimetric samples made con- 
currently with instantaneous sampling at 
this longwall yielded 0.3 mg/m-^ of dust 
generated from support movement. 

Dust concentrations produced by the 
supports at longwall B were similar to 
those produced at longwall A; the average 
instantaneous and gravimetric concentra- 
tions were 0.4 RAM unit and 0.1 mg/m-^ , 
respectively. Besides the similarities 
in support-generated dust concentrations, 
longwalls A and B had many similarities 
with respect to mining conditions , sup- 
port design, and operational procedures. 



GEOLOGY 

The mining conditions at longwalls A 
and B were very similar. Longwall A was 
located in the Lower Kittanning coal 
seam, with a mining height of about 5 ft. 
The mining face was horizontal, with no 
major irregularities in the coal seam. 
The immediate roof was composed of 48 ft 
of dark-to-medium-gray hard shale. This 
roof was very competent in front of the 
canopy tips and caved readily at the rear 
of the supports. Its assumed compressive 
strength was 10,000 psi. The floor was a 
dark-to-medium-gray claystone of excel- 
lent quality. The cover over the panel 
was 320 ft deep and was formed of thick 
sandstone and shale strata interlaced 
with thin coal seams. 

Longwall B was located in the Campbell 
Creek coal seam. The mining height was 
approximately 6 ft. The coal seam had no 
major irregularities and was relatively 
flat. The immediate roof consisted of 



5 ft of a gray hard shale overlain by 
32 ft of gray sandstone. This roof be- 
haved very well, being competent in front 
of the canopy tips and caving readily be- 
hind the supports. (Its compressive 
strength was also 10,000 psi) . The floor 
was a gray sandstone of excellent quality 
which provided a firm surface for the 
support bases. The cover over the panel 
was 500 ft deep and was composed of thick 
sandstone and shale strata interlaced 
with coal. 

Thus, both longwalls had excellent min- 
ing conditions. 

SUPPORTS 

The supports used at the two longwalls 
were identical: four-legged lemniscate 
shield supports, each with a 55-ft^ can- 
opy bearing area. The setting pressures 
and yield loads of these supports were 
4,350 pslg and 564 tons, respectively. 
The side seals on the flushing shield 
were spring-activated with hydraulic 
override. The supports could be moved in 
the contact-advance mode from a bidirec- 
tional adjacent control. 

The strong shale roof at both longwalls 
was assumed to have had a compressive 
strength of 10,000 psi. The load den- 
sity set on the roof by the support at a 



setting pressure of 4,350 psig was 113.9 
psi, resulting in a roof loading fac- 
tor of 87.7 (roof compressive strength/ 
load density at set pressure) at both 
longwalls. This roof loading factor in- 
dicated that the roof had a strong re- 
sistance to crushing under the setting 
loads of the supports. 

OPERATIONAL PROCEDURES 

Both longwalls A and B used a unidirec- 
tional cutting sequence with contact ad- 
vance of supports in one of the cutting 
directions. Also, each mine maintained 
good horizon control during mining. Con- 
tact advance and good horizon control re- 
duce the amount of debris that is re- 
ground and crushed into respirable dust. 

Also, both longwalls applied water to 
the roof to reduce dust entrainment when 
supports were moved. Longwall A free- 
wheeled the leading drum near the roof 
during the cleanup pass to apply water. 
Longwall B wet the roof using a venturi 
spray mounted on the shearer body and 
directed downstream at an angle of ap- 
proximately 45° relative to the roof. 
The average velocity of the air at the 
face, traveling in a head-to-tail direc- 
tion, was 580 ft/min for longwall A and 
289 ft/min for longwall B. 



LONGWALLS WITH MODERATE CONCENTRATIONS OF SUPPORT-GENERATED DUST 



At four of the longwalls surveyed 
(longwalls C, D, E, and F) , moderate 
amounts of respirable dust were generated 
by the supports. Average respirable dust 
concentrations measured with gravimetric 
samplers were between 0.5 and 2.0 mg/m-'. 
Average respirable dust concentrations 
measured with the RAM's ranged from 0.6 
to 2.4 RAM units. 

A typical example of instantaneous 
dust concentrations measured at a long- 
wall with moderate amounts of support- 
generated dust (longwall F) is shown in 
figure 3. Again, the difference between 
the immediate downwind and upwind concen- 
trations measured during support movement 
(the shaded area) represented the dust 
generated by the supports . Peak support 
dust concentrations were measured at ar- 
eas of the face with deteriorated roof 
conditions. The average downwind dust 
concentration was 2.0 RAM units, and the 



average upwind dust concentration was 0.3 
RAM unit, resulting in an average dust 
concentration of 1.7 RAM units generated 
by the supports. The corresponding dif- 
ference between the downwind and upwind 
gravimetric sample concentrations yielded 
an average dust concentration of 1.5 
mg/m-^. The other longwalls (C, D, and E) 
also had moderate dust concentrations, 
and instantaneous peak concentrations 
similar to those measured at longwall F 
were measured at deteriorating roof areas 
along their faces. 

GEOLOGY 

Longwalls C, D, E, and F were located 
in three different coal seams , but had 
similar mining conditions. Longwall C 
was located in the 6-ft-thick Eagle Seam. 
Longwall D was located in the 6-ft-thick 
Pittsburgh Seam. Longwalls E and F were 




20 30 40 50 60 70 80 

-• — Headgate Tai Igate *■ 

SUPPORT MOVED 
(Support identification number) 

FIGURE 3. - Instantaneous roof support dust concentrations at longwall F. 



located in the 9-ft-thick Blind Can- 
yon Seam. No major seam irregularities 
occurred at any of these longwalls except 
at longwall D, which had 1 ft of rock 
parting in the middle of the seam. Each 
of the longwalls had a fairly soft fria- 
ble roof composed of shale (longwall C) , 
coal (longwall D) , or siltstone (long- 
walls E and F) . The floor at longwalls C 
and D was a wet, soft shale; at longwalls 
E and F, the floor was a wet, soft 
muds tone. 

Depths of cover over the longwall 
panels ranged from 430 to 1,600 ft. In 
general, the longwalls under greater 



depths of cover had somewhat higher 
concentrations of support-generated dust. 
The stratigraphic composition of the 
overburden for longwalls C, D, E, and F 
consisted mainly of shales, sandstones, 
limestones, siltstones, and coal seams. 
General mining conditions for these 
longwalls were fair to good. 

SUPPORTS 

All four longwalls used two-legged lem- 
niscate shields that were basically 
similar in design, but with some differ- 
ences. The shields used at longwalls C, 



E, and F had extendable forepoles. The 
canopy areas with the forepoles retracted 
were 42.0, 42.8, and 42.8 ft2, respec- 
tively; with the forepoles extended, they 
were 52.7, 59.0, and 59.0 ft^, respec- 
tively. The shields used at longwall D 
had no forepoles and had a canopy area of 
50.7 ft^. The setting pressure for long- 
walls C, E, and F as 4,350 psig; for 
longwall D, it was 4,000 psig. Yield 
loads at longwalls C, D, E, and F were 
426, 352, 472, and 472 tons, respective- 
ly. The canopy areas with the forepoles 
retracted were used for the load-density 
calculations for longwalls C, E, and F 
because the forepoles were not extended 
on most of the shields at these faces. ^ 
The side seals on the flushing shields 
at these longwalls were spring-loaded 
with hydraulic override, and the sup- 
ports could be moved in the contact- 
advance mode from a bidirectional adja- 
cent control. 

Loading densities exerted on the roofs 
at longwalls C, D, E, and F were 134.7, 
40.3, 108.3, and 108.3 psi, respectively. 
At longwall D, the load density on the 
roof was significantly lower because of 
the fairly large canopy area and smaller 
props on the longwall D shields. The 
roof loading factors (roof compressive 
strength/ average set load density) for 
longwalls C, D, E, and F were 29.7, 16.1, 
36.9, and 36.9, respectively. These fac- 
tors were significantly lower than the 
roof loading factors of longwalls A and 
B, indicating that the roofs over long- 
walls C, D, E, and F were more suscepti- 
ble to crushing and grinding during sup- 
port movement. 



OPERATIONAL PROCEDURES 

Each of these four mines advanced the 
supports during the head-to-tail pass. 
Longwalls E and F did not utilize contact 
advance; longwalls C and D utilized con- 
tact advance in some areas of the face 
where the floor was strong enough to keep 
the supports from digging in. 

At all four longwalls, the floor and 
roof were cut fairly evenly. However, 
the supports would sink or dig into the 
floor, and some of the weaker roof areas 
would break off in front of the canopy 
tips, leaving cavities on top of the can- 
opies. These cavities developed highly 
stressed roof-contact areas that frac- 
tured and crushed. When the supports 
were dropped significantly before being 
advanced, a thick layer of debris would 
build up on the canopy, and this debris 
was subject to further crushing and 
grinding during set loading. At long- 
walls C and D, the support movers lowered 
the front of the canopy and cleaned the 
debris off some of the shields into the 
panline. This was a very dusty operation 
and was conducted upwind of the shearer 
operators and other face personnel. If 
debris needs to be cleaned off the can- 
opies , it should be done downwind of all 
workers. One longwall (longwall F) uti- 
lized spray manifolds on the shields to 
suppress support dust. 

Ventilation ^airflow was less than 200 
ft/min at three of these longwalls. 
Longwalls C, E, and F had average face 
air velocities of 167, 184, and 179 ft/ 
min. Longwall D had an average face air- 
flow of 402 ft/min. 



LONGWALLS WITH HIGH CONCENTRATIONS OF SUPPORT-GENERATED DUST 



At two of the longwalls surveyed (long- 
walls G and H) , large amounts of res- 
pirable dust were generated by the sup- 
port movement, with average respirable 
dust concentrations above 2.0 mg/m^ 

'The forepoles would be utilized mainly 
to catch loose material separated from 
the roof in some areas of the face. 
Therefore, there was probably very lit- 
tle if any loading of the roof with the 
forepoles. 



as measured with gravimetric samplers. 
Average respirable dust concentrations 
measured with the RAM exceeded 2.0 RAM 
units. 

A typical example of instantaneous dust 
concentrations at a longwall with large 
amounts of support-generated dust (long- 
wall G) is shown in figure 4. Again, the 
shaded area between the two curves repre- 
sents the dust generated by supports. 
The average downwind and upwind RAM 
measurements were 6.1 and 0.6 RAM units. 



< 



< 
cc 



LU 
O 

z 
O 
o 

I- 
co 

ID 
Q 

CO 

3 

o 

LXJ 



12 



10 



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J — I — I — I I 1 it I I I I I I I I I I I I I I I I I I 



10 20 30 

-• Headgate 



50 60 70 80 

Tailgate — 
SUPPORT MOVED 
(Support identification number) 



90 



FIGURE 4. - Instantaneous roof support dust concentrations at longwall G. 



respectively. The amount of dust gener- 
ated by the supports varied significant- 
ly along the face. The highest and low- 
est dust concentrations measured dovmwind 
of the supports were 11.8 and 2.2 RAM 
units, respectively. Less dust was gen- 
erated by the supports at longwall H, but 
the amount of dust generated was still 
significant. The average RAM and gravi- 
metric measurements of support-generated 
dust for both longwalls were 2.6 RAM 
units and 3.0 mg/m^. Instantaneous RAM 
measurements showed that support dust 
concentrations varied at longwall H but 
were more consistent than at longwall G. 

GEOLOGY 

Mining conditions at longwalls G and 
H were similar. Longwall G was located 
in the Hiawatha coal seam, with a min- 
ing height of about 8 ft. The mining 
face was horizontal, with no major 



irregularities in the coal seam. The 
immediate roof was composed of 1 ft of 
coal overlain with 10 ft of sandstone. 
This roof was fairly competent in front 
of the canopy tips and caved readily at 
the rear of the supports. The assumed 
compressive strength of the coal roof was 
650 psi. The floor was a dry sandstone 
of excellent quality and provided a firm 
surface for the support based. Overlying 
this longwall panel was 1,600 ft of over- 
burden composed of sandstones, mudstones, 
siltstones, shales, and coal seams. 

Longwall H was located in the E-Seam. 
This coal seam is characterized by local 
thickening and thinning due to the pres- 
ence of rolls in the coalbed. The strata 
above the coal seam are composed of sand- 
stone, shale, and mudstone. The seam is 
fairly flat, with a 2° to 3° dip in the 
northern direction. At the longwall pan- 
el, 8 ft of coal was mined, limited by 
the maximum support height , leaving 1 ft 



of coal for the immediate roof below the 
sandstone strata above the seam. This 
roof was fairly competent in front of the 
canopy tips and caved readily at the rear 
of the supports. The assumed compressive 
strength of the coal roof was 650 psi. 
The floor was a hard, dry shale and pro- 
vided a firm surface for the support 
bases. Overlying this longwall panel was 
2,200 ft of overburden composed of sand- 
stones, mudstones, siltstones, shales, 
and coal seams . 



Since the immediate roof at both long- 
walls was coal, it was assumed that the 
roof at both mines had a compressive 
strength of 650 psi. Load densities 
exerted on the roof by the supports at 
longwalls G and H were 73.6 and 80.6 psi, 
yielding roof loading factors of 8.8 and 
8.1, respectively, indicating a tendency 
of the roof to crush and grind under set- 
ting loads . 

OPERATIONAL PROCEDURES 



SUPPORTS 

The supports used at longwalls G and 
H were similar in design. Supports at 
longwall G were two-legged lemniscate 
shields with a 57.6-ft^ canopy bearing 
area. The setting pressures and yield 
loads of the supports were 4,350 psig and 
472 tons, respectively. Longwall H also 
had two-legged lemniscate shields, but 
the canopy bearing area was 54.0 ft^, the 
setting pressure was 4,500 psig, and the 
yield load was 440 tons. At both long- 
walls, the side seals on the flushing 
shields were spring-activated with hy- 
draulic override. The supports could be 
moved in the contact-advance mode from a 
bidirectional adjacent control. 



Longwalls G and H used a unidirectional 
cutting sequence with contact advance of 
supports during the head-to-tail pass. 
Horizon control was maintained fairly 
well. Ventilation at G and H was head- 
to-tail, with average face airflows of 
650 and 355 ft/min, respectively. Mining 
conditions at these longwalls were fairly 
good. The large amounts of roof support 
dust seemed to be generated by the easy 
crushing and grinding of the coal roof 
during advance and setting of the sup- 
ports. The airflow at longwall G was 
quite high, which could have contributed 
to the entrainment of support-generated 
dust. 



FACTORS AFFECTING DUST GENERATED BY SUPPORTS 



Eight longwalls were surveyed and cate- 
gorized according to the amount of res- 
pirable dust generated by roof supports 
(low, moderate, and high). Within these 
categories, similarities were observed, 
mainly with respect to mining conditions 
(geology). Mining conditions determined 
by the coalbed geology seemed to be the 
main factors affecting support dust 
generation. 

ROOF STRENGTH 

Strong shale roof seemed to generate 
the least amount of dust during support 
movement (< 0.5 mg/m^). The weaker shale 
and siltstone roofs generated moderate 
amounts of dust during support movement 
(> 0.5 mg/m? and < 2.0 mg/m-'). Longwalls 



that left an immediate coal roof (lowest 
compressive strength) because of weak 
strata above the coal or limits of the 
longwall equipment had the highest 
amounts of support-generated dust (> 2.0 
mg/m^). Thus, there appears to be an in- 
verse relationship between roof strength 
and support-generated dust. Figure 5 
shows the relationship of the roof load- 
ing factor to dust concentration. Usual- 
ly, the lower the roof -loading factor 
(roof compressive strength/load density 
exerted on roof by canopy) , the weaker 
the roof. The dust concentrations plot- 
ted in figure 5 were the average RAM mea- 
surements of support dust at each long- 
wall; using the gravimetric dust data 
instead of the RAM measurements yielded 
the same relationship. 



10 



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1 


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- 







20 



40 



60 



80 



100 



ROOF LOADING FACTOR 



FIGURE 5. - Relationship between roof loading factor and average support dust concentration at 
surveyed longwall faces. 



DEPTH OF COVER 

Another geologic factor that seemed to 
affect the amount of support-generated 
dust was the depth of cover over the coal 
seam. At the eight longwalls studied, 
the support-generated dust seemed to in- 
crease as overburden above the seam in- 
creased. In figure 6, seam depth is 
plotted against average RAM concentra- 
tions of the support dust at each long- 
wall. The gravimetric data for these 
longwalls showed the same trend. As 
depth of cover increases, vertical and 
horizontal stresses in the strata in- 
crease, usually producing more pronounced 
fracturing of the strata. Also, deeper 
strata usually have more in situ stresses 
in all directions. Generally, fractured 
and highly stressed rock, strata are weak- 
er and less competent during mining, 
which may make them more susceptible to 
crushing and grinding by longwall roof 
supports. 

SHIELD DESIGN 

The longwall roof supports at the eight 
longwalls were either two- or four-legged 
lemniscate shields. Their designs did 
not seem to be as strong a factor in dust 
generation as roof strength. The only 



major difference between the two-legged 
and four-legged shields was that the 
four-legged shields distributed floor 
loads more evenly throughout the bases. 
Other differences between the shields 
were their prop sizes and canopy areas, 
which produced different load densities 
exerted on the roof with approximately 
the same setting pressures. However, 
there seemed to be no relationship be- 
tween the load densities alone and 
support-generated dust. A relationship 
did appear when the load densities exert- 
ed on the roof and the roof strength were 
utilized together to determine the roof- 
loading factor. 

OTHER FACTORS 

Other factors that may influence the 
generation of support dust are horizon 
control, contact advance, water applica- 
tion, and face airflow. Good horizon 
control should be maintained by the 
shearer or plow. When irregularities 
occur in the roof and floor, the bearing 
areas of the supports make contact with 
only a portion of the roof or floor and 
will crush out these areas due to high 
stress. All the longwalls included in 
this study maintained good horizon con- 
trol. However, at longwalls C, D, E, and 



11 



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g "200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000 2,200 

PANEL DEPTH, ft 

FIGURE 6. - Relationship between panel depth and average support dust concentration at surveyed 
longw/all faces. 



I 



F, the roof broke off in front of the 
canopies, leaving cavities in the roof 
and above the support canopies after they 
were advanced. The smaller contact area 
between the roof and support canopies was 
highly stressed and fractured. When the 
support was lowered to be moved, debris 
accumulated on the canopy and was further 
crushed under setting loads, producing 
respirable dust. To reduce debris build- 
up, contact advancement of the supports 
should be utilized to scrape the debris 
off the canopies as the supports are 
advanced. 

Water application on the immediate roof 
may also help reduce support-generated 
dust. The residual moisture on the roof 
should suppress some of the dust during 
the crushing and grinding action of roof 
supports. At longwalls A and B, which 
had the least support-generated dust, the 
roof was wet with the shearer. However, 



it was difficult to determine what effect 
the water had on the roof support dust 
because these longwalls had the strongest 
roofs and the best mining conditions. 

Airflow can also affect dust concen- 
trations. If the air velocity along 
the face is maintained in a moderate 
range, from 350 to 600 ft/min, good dust 
diffusion and dilution can be achieved 
some distance downwind from the generat- 
ing source, as shown in figure 7 (_3 ) . 
Below this range, dust levels can be sig- 
nificantly higher because of inadequate 
dilution and diffusion. Above this 
range, dust levels can also be signifi- 
cantly higher because of dust entrainment 
at higher airflows. However, this trend 
was not observed in the present study be- 
cause support dust measurements were made 
adjacent to support movement, not allow- 
ing sufficient time for diffusion and 
dilution. 



SUPPORT DUST CONTROL TECHNOLOGY USED IN THE UNITED STATES 



Several support dust control techniques 
were observed to be in use or on trial in 
the United States. These techniques in- 
volve water application and/or support 



movement practices. The effectiveness of 
some of these techniques is unknown, and 
some of them may be impractical for cer- 
tain longwall operations. 



12 



AVERAGE FACE AIR VELOCITY, m/s 

2 3 4 




200 400 600 800 

AVERAGE FACE AIR VELOCITY, ft/min 
FIGURE 7. - Relationship between face air velocity and dust levels at the face (3). 



000 



WATER APPLICATION 

Several water application techniques 
for longwall support dust control cur- 
rently practiced in U.S. coal mines are 
discussed briefly below. 

Support Washdown . - One shearer opera- 
tor uses a shearer water hose to hose 
down the supports and roof during the 
head-to-tail or tall-to-head pass. The 
residual moisture reduces dust entrain- 
ment when the supports are moved. For 
this procedure to be effective, suffi- 
cient water must be supplied to the 
shearer so that its internal and external 
water systems are not affected during 
washdown. 

Wetting Immediate Roof With Shearer 
Water . - This can be done in several 
ways. One way is to maintain suffi- 
cient water pressure at the drum sprays 
to wet the roof while it is being cut. 
Also, during the cleanup pass in a uni- 
directional operation, the lead drum, 
which typically will not be cutting much 
material, can be freewheeled near the 
roof, allowing the water sprays to wet 
the roof. Another alternative is to 
spray the unsupported roof using one or 



several water sprays mounted on top of 
the shearer body, directing the water 
with the airflow and upward at an angle 
of approximately 45°. 

Mounting Water Sprays on Support Cano- 
pies Over Panline . - Water sprays can be 
mounted on the support canopies in sev- 
eral ways. Usually several sprays are 
directed down at the face and in the 
direction of the airflow from a manifold 
at every tenth support (fig. 8). Their 
purpose is to humidify the face area to 
suppress support dust generation. The 
suppression effectiveness of these sprays 
has not been documented; however, studies 
indicate that they actually move air 
and help diffuse and dilute the dust 
from supports. Drawbacks are that these 
sprays are difficult to maintain and tend 
to wet the face personnel. 

OTHER DUST CONTROL PRACTICES 

Where water application might deterio- 
rate the roof and floor, the practices 
described below are employed during sup- 
port movement to reduce the dust exposure 
of face workers; these practices can also 
be used in conjunction with water. 



13 




FIGURE 8. - Spray manifold located on support canopy. 



Minimizing Debris on Top of Canopies . - 
Debris on top of shield canopies can be 
minimized by contact advancing supports 
(shields) so that the debris can be 
scraped off the canopies into the gob. 
This avoids crushing and grinding of de- 
bris during support movement and thereby 
generally reduces resplrable dust genera- 
tion (fig. 9) (4^). Also, maintaining 
good horizon control during mining yields 
more contact area between the support 
canopy and the roof , which reduces highly 
stressed areas on the roof and minimizes 
spaces or cavities where debris can build 
up. 

Advancing Supports During Pass Cycle 
Against Airflow . - This practice can be 
employed on unidirectional operations , 
allowing all face personnel to work on 
the intake-air side of support advance. 
When the pass cycle against airflow is 



used as a cleanup pass, dust levels gen- 
erated by the shearer are low, and al- 
though the support movers are on the 
return-air side of the shearer, the dust 
exposure levels they are subjected to can 
be kept low. When the primary cut is 
taken against the airflow, a properly de- 
signed external shearer water-spray sys- 
tem (shear clearer) can confine shearer- 
generated dust against the face for 
approximately 40 ft (fig. 10), thus main- 
taining an acceptable dust-exposure level 
for the support movers immediately down- 
wind of the shearer (_5) . 

Diluting and Diffusing Support Dust 
When Supports Are Advanced With Air- 
flow . - It is not always possible to 
advance supports only on the return-air 
side of the shearer. Roof conditions 
may necessitate support advance immedi- 
ately after the shearer cuts the face. 



14 




1.0 1.5 2.0 

DEBRIS THICKNESS ON CANOPY, in 

FIGURE 9. - Relationship between debris thick- 
ness on shield canopy and dust generated (4). 



Bidirectional cutting requires support 
advance during both directional passes. 
In some operations, it may not be possi- 
ble to install an external water-spray 
system capable of confining the dust 
against the face for any substantial dis- 
tance downwind of the shearer. Under 
these circumstances, the most feasible 
method of dust control is to dilute and 
diffuse the dust by increasing the dis- 
tance between support movement and the 
shearer. Figure 11 shows how dust levels 
in the walkway decrease with increases in 
distance downwind from support movement. 
Based on the walkway dust levels shown in 
figure 11, it is recommended that a dis- 
tance of at least 50 ft be maintained 
between support movement and the shearer. 
Increasing the face airflow also promotes 
dilution and diffusion (6^). However, 
face velocities should not exceed 600 
ft/min, to prevent dust entrainment. 



< 



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z 
o 

O m 

51 



D 
Q 
CO 

o 

LU 

z 
< 

Z 
< 

CO 



10 
8 
6 
4 
2 




Direction of airflow 

< 

Direction of cut 



Without shearer clearer 



50 40 30 20 
INTAKE SIDE, ft 




Shearer 

FIGURE 10. - Effectiveness of shearer-clearer system 



20 30 40 50 
RETURN SIDE, ft 



15 




10 20 30 40 50 60 

DISTANCE DOWNWIND OF SUPPORT ADVANCE, ft 

FIGURE 11. - Relationship between distance downwind of support movement and dust level. 



CONCLUSIONS 



I 



Although several factors can affect the 
amount of respirable dust generated by 
longwall supports, this study indicates 
that geologic conditions (roof conditions 
and depth of cover) are the most signifi- 
cant factors. The amount of support dust 
generated was inversely related to roof 
strength and directly related to depth of 
cover over the longwall panel. Some 
techniques for controlling support dust 
generations are currently in use. Al- 
though some of these techniques have lim- 
itations, they should be applied if 
support-generated dust makes up a signif- 
icant portion of the face personnel's 
overall dust exposure. 

Further research and development are 
needed to advance longwall support dust 
control technology, to improving effi- 
ciency and to overcome the limitations of 
the existing techniques. The objective 



of this research should be improved sup- 
port design. One area that needs to be 
addressed is reducing stresses on host 
strata from supports while maintaining 
acceptable roof convergence. Certain 
roof types are more susceptible to crush- 
ing and grinding from longwall roof sup- 
ports , and therefore they generate more 
dust during support movement. An im- 
proved seal between supports needs to be 
developed to prevent the dust from the 
roof and gob areas from entering the face 
area. It appears that the side seals 
presently used on supports allow a sig- 
nificant amount of dust to seep into the 
face area from the gob and roof. Final- 
ly, a reliable automated support advance 
system would allow all face personnel to 
be positioned upwind of the dust gener- 
ated by support movement. 



16 



REFERENCES 



1. Jankowski, R. A., and J. A. Organ- 
iscak. Dust Sources and Controls on the 
Six U.S. Longwall Faces Having the Most 
Difficulty Complying With Dust Standards. 
BuMines IC 8957, 1983, 19 pp. 

2. Peters, W. C. Exploration and Min- 
ing Geology. Wiley, 1978, p. 159. 

3. Mundall, R. L. , R. A. Jankowski, 
T. F. Tomb, and R. S. Ondrey. Respi- 
rable Dust Control on Longwall Mining 
Operations in the United States. Pa- 
per in Proc. 2d Int. Mine Ventilation 
Symp., Nov. 2-8, 1979, Reno, NV, 
9 pp.; available upon request from Univ. 
Reno. 



4. National Coal Board (London). Re- 
port on Visit to Houillleres du Bassin de 
Lorraine, Freyming Merebach, for Meeting 
of Sub-Committee on Dust From Roof Sup- 
ports. Visit Rep. (82)7, Nov. 17-19, 
1982, 5 pp. 

5. U.S. Bureau of Mines. Reoriented 
Shearer Water Sprays Move Dust Toward 
Face. Technol. News 112, Oct. 1981. 

6. . Reduce Dust on Longwall 

Faces With a Gob Curtain. Technol. News 
119, Nov. 1981. 



7. 



Improved Respirable Dust 



Monitor. Technol. News 72, Oct. 1979. 



APPENDIX A. —DATA FROM LONGWALL SURVEY 



TABLE A-1. - Numerical data from survey 



17 



Longwall ' 



B 



D 



E 



H 



Seam thickness ft.. 

Seam depth f t. . 

Roof strength (est) psi.. 

Canopy area f t^. . 

Setting pressure pslg. . 

Av set load density on roof 

tons/ft2. . 
psi.. 

Roof loading factor^ 

Yield pressure psig. . 

Yield load tons. . 

Av face air velocity f t/min. . 

Av support dust cone along face: 

RAM RAM units . . 

Gravimetric mg/m^ . . 

Av Average. cone Concentration 
^ See table A-2 for description of 
^Average roof strength/ average set 



5 

320 

10,000 

55.0 

4,350 

8.2 

113.9 

87.7 

5,440 

564 

580 

0.4 
0.3 



6 

500 

10,000 

55.0 

4,350 

8.2 

113.9 

87.7 

5,440 

564 

289 

0.4 
0.1 



6 

1,320 

4,000 

42.0 

4,350 

9.7 

134.7 

29.7 

4,570 

426 

167 

0.8 
1.7 



6 

430 

650 

50.7 

4,000 

2.9 

40.3 

16.1 

9,580 

352 

402 

0.6 
1.0 



9 

1,600 

4,000 

42.8 

4,350 

7.8 

108.3 

36.9 

6,150 

472 

184 

2.4 
1.3 



9 

1,600 

4,000 

42.8 

4,350 

7.8 

108.3 

36.9 

6,150 

472 

179 

1.7 
1.5 



1,600 

650 

57.6 

4,350 

5.3 
73.6 
8.8 
6,730 
472 
650 

5.5 
10.7 



8 

2,200 

650 

54.0 

4,500 

5.8 
80.6 
8.1 
6,300 
440 
355 

2.6 
3.0 



est Estimated, 
longwalls. 
load density. 



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33 



APPENDIX B.— DUST-SAMPLING STRATEGY^ 



19 



Support-generated dust was sampled with 
both instantaneous and gravimetric sam- 
plers. The instantaneous sampler used 
was the GCA Real-Time Aerosol Monitor 
(RAM). This is a light-scattering in- 
strument that measures the volume of res- 
pirable dust in a volume of air instan- 
taneously (_7 ) . Its numerical output is 
in RAM units, which approximate respira- 
ble dust concentrations in milligrams per 
cubic meter. The gravimetric samplers 
consisted of a pump, cylone, and filter: 
A Dupont model P-2500 pump drew 2 L/min 
of air through a 10-mm Dorr-Oliver nylon 
cyclone and deposited the respirable dust 
on a 5-ym MSA preweighed cassette filter. 
This sampler gave an average weight per 
unit volxome of air in milligrams per cu- 
bic meter. 

The amount of respirable dust generated 
by support movement was measured by 
two people, each using one RAM and two 
gravimetric samplers. Samples were taken 
on the immediate upwind side of each 

^Dust concentrations were measured only 
during support movement and cannot be 
directly related to an 8-h average expo- 
sure for compliance purposes. 



support moved and on the immediate down- 
wind side of each support moved. Sam- 
pling followed support movement along the 
face, with gravimetric samplers continu- 
ously running and RAM measurements taken 
at every other support. The difference 
between the downwind concentrations and 
the upwind concentrations indicated the 
amount of dust generated by supports 
(shaded areas between curves in figures 
2, 3, and 4). 

RAM measurements were also made at some 
faces to determine the effect of ventila- 
tion on the diffusion and dilution of 
support dust. These measurements were 
made in an area of the face during move- 
ment of about 12 supports. Sampling was 
conducted from a stationary position 
downwind of all the supports to be moved. 
During the advance of each support , the 
dust concentration at the stationary po- 
sition was measured and the distance from 
the support moved was recorded. The con- 
centrations measured at various distances 
from the supports were used to show how 
support-generated dust is diluted and 
diffused by face ventilation, yielding 
lower dust concentrations at greater dis- 
tances downwind of support movement. 



i!rU.S. CPO: 1985-505-019/20,040 



INT.-BU.OF MINES, PGH., PA. 27953 



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